I believe that they're popular mainly because well it was the first of all cryptocurrencies to be made. Honestly most of the other coins are scams and greedy ways that the developers make money. The only other coin I can think of that isn't a scam are dogecoins! What do you think?
because bitcoin Bitcoin is amazing! Theres a limit to how many will ever exist and significant control over my own money. And Even if Bitcoin continues to be the most valuable cryptocurrency, there may be room for altcoins to serve some other niche and hence may not be bad bets in their own right.